Last updated: August 3, 2002
Canon introduced wireless flash for the EOS system in 1998 with the 550EX speedlite and transmitter ST-E2. When used with a Canon A-type body, the system is capable of wireless E-TTL, FP Flash and FEL automatic flash exposure and manual flash exposure. (Canon B-type bodies are capable only of wireless manual flash exposure.) The system is designed such that a single master unit connected to the camera hot shoe controls any number of remotely positioned slave units. Four different channels ("1", "2", "3" or "4") are selectable to lessen the chance of interference between other photographer's setups. Speedlites can be set to one of three groups ("A", "B", or "C") for lighting ratio control.
Master-Slave Communications
The master unit communicates to the slaves with either a visible light
or near-infrared signal (depending on the type of master unit) and must
be positioned such that the signal can be "seen" by the slaves. Slave
units must be positioned such that their front side can receive the master's
signal. In practice, these signals propagate well indoors, reflecting
off the ceiling and walls. Pressing the "pilot" lamp button on the
master will trigger each slave unit to fire a short pulse (1/64 power);
all "A" group slaves firing together, followed by the "B" slaves and finally
the "C" slaves.
Flash Ratio Control
Each slave speedlite has to be designated a member
of either group "A", "B" or "C". The 550EX, MR-14EX or MT-24EX master
speedlite can set the A : B lighting ratios between 8:1 and 1:8 in thirteen
1/2 stop steps and set exposure compensation for the "C" group from -3
to +3 in 1/3 stop steps. Flash ratio control is possible only with
the EOS 3 or 1V.
Modeling Light
When the Depth of Field Preview button on the
EOS 3, 1V, D30, D60 or 1D is pressed (these bodies only), each speedlite
fires a 70Hz burst for one second, enabling the photographer to
visually see the lighting effects. The modeling light feature can be disabled
with a custom function.
Canon has introduced 5 wireless flash components:
Speedlite 550EXA-Type Camera Bodies
The 550EX can serve as either a "normal" speedlite and/or as a master or slave unit. The master speedlite flash can be disabled. The 550EX can control 3 groups of slaves for ratio control, A : B ratio and (A : B) : C ratio. Flash exposure compensation can be individually set on each 550EX. Communications with slave units is with visible light.
http://www.usa.canon.com/html/cameras_speedlite/550ex.htmlSpeedlite 420EX
The 420EX can serve as either a "normal" speedlite or as a slave unit.
http://www.usa.canon.com/html/cameras_speedlite/420ex.htmlMacro Ring-lite MR-14EX
The MR-14EX can serve as either a "normal" speedlite and/or as a master unit. The MR-14EX can control 3 groups of slaves for ratio control. Both halves of the ring light can be controlled as groups "A" and "B" as can be any number of wireless slaves. Communications with slave units is with visible light.
http://www.usa.canon.com/html/cameras_speedlite/mr14.htmlMacro Ring-lite MT-24EX
The MT-24EX can serve as either a "normal" speedlite and/or as a master unit. The MT-24EX can control 3 groups of slaves for ratio control. The removeable flash heads can be controlled as groups "A" and "B" as can be any number of wireless slaves. Communications with slave units is with visible light.
http://www.usa.canon.com/html/cameras_speedlite/mt24ex.htmlTransmitter ST-E2
The ST-E2 can serve only as a master unit. The ST-E2 has no flash output but has all other "master" capabilies including an AF-assist lamp. The ST-E2 can control only 2 groups of slaves for ratio control ("A" and "B" only) and exposure bracketing is not possible. Communications with slave units is with near-infrared light.
http://www.usa.canon.com/html/cameras_speedlite/550ex.html